1. Field of the invention:
This invention relates to a hydraulic system, and more particularly to an integrated hydraulic system for use in construction work vehicles such as, for example, tire shovel loaders, dump trucks, motor scrapers etc. The hydraulic system arranged to supply hydraulic fluid delivered by three hydraulic pumps, that is, a pump for supplying hydraulic fluid to operate the steering mechanism, an auxiliary pump and a pump for supplying hydraulic fluid to operate work implements, into steering mechanism operating actuator and work implement operating actuators.
2. Description of the prior art:
Most of the construction work vehicles are provided with a hydraulic system wherein the steering mechanism and the work implements are arranged to be operated, respectively, by separate hydraulic circuit systems independent of each other.
On the other hand, the hydraulic system disclosed in Japanese Patent Laid-Open Publication No. 57-114761 provisionally published on Jul. 16, 1982 comprises a fixed displacement hydraulic pump used exclusively for operating the steering mechanism, a fixed displacement hydraulic pump used exclusively for operating work implements and an auxiliary fixed displacement hydraulic pump, all of which are driven by the propulsion engine mounted on the vehicle, the arrangement being made such that the direction of flow of hydraulic fluid delivered by the three hydraulic pumps can be controlled by a hydraulic circuit having a single pilot operated directional control valve installed therein. For example, when the engine is operating at a low speed near low idling speed, the flow rate of the hydraulic fluid delivered by the pump used exclusively for operating the steering mechanism is less than that required to operate the steering mechanism, and therefore, the hydraulic fluid delivered by the auxiliary hydraulic pump is allowed to join that delivered by the hydraulic pump used exclusively for operating the steering mechanism to thereby supply a required flow rate of hydraulic fluid into the steering mechanism, thus enabling steering operation to be conducted quickly and reliably. Further, when the engine is operating at a high speed, the flow rate of the hydraulic fluid supplied by the hydraulic pump used exclusively for operating the steering mechanism is high enough to operate the steering mechanism, and therefore the hydraulic fluid delivered by the auxiliary hydraulic pump is supplied into work implement operating actuators. Still further, the arrangement is made such that when the engine is operating at a medium speed, the hydraulic fluid delivered by the auxiliary hydraulic pump is supplied or distributed through a restrictor means into the steering mechanism and the work implement operating actuators, respectively. The above-mentioned conventional hydraulic system is however disadvantageous in that, when the engine is rotating at an extremely low speed near low idling speed, in order to supply the sufficient flow rate of hydraulic fluid into the steering mechanism, it is necessary to envisage the increase of the capacity of the steering pump and the auxiliary pump, however in that case it becomes impossible to achieve a major proposition that the power output developed by the engine is utilized effectively for running of the vehicle provided with hydraulic system comprising hydraulic pumps with limited capacities.
Further, disclosed in the U.S. Pat. No. 3,355,994 (issued to Thomas J. Malott) is a hydraulic system arranged to conduct the steering operation and the work implement operation by using three hydraulic pumps driven by an engine. In this hydraulic system, there is also a disadvantage as in the case of the foregoing Japanese Patent Laid-Open Publication No. 57-114761. That is, in order to supply the sufficient flow rate of hydraulic fluid into the steering mechanism so as to effect a preferable steering operation when the engine is operating at an extremely low speed near the low idling speed, it will have to provide a large sized steering pump and a large sized auxiliary pump so as to permit them to increase their respective pump capacities.